<title>555 Missing Pulse Detector</title>
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<applet code=Circuit.class archive=circuit.jar width=740 height=450>
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<param name=pause value=20>
<param name=startCircuit value="555missing.txt">
<param name=startLabel   value="555 Missing Pulse Detector">
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<p>
This circuit uses a
<a href="http://en.wikipedia.org/wiki/555_timer_IC">555 timer
chip</a> to
detect a missing pulse.  The input is a square wave.  When you click
the logic input (the "L") to bring it high, then the input will stop
oscillating and stay high.  The 555 will detect this and bring the output low.
<p>
The 555 is wired as a <a href="e-555monostable.html">monostable</a>.
But
whenever the input goes low, it turns on a transistor which quickly drains
the capacitor.  When the input goes high,
the capacitor recharges, but doesn't quite make it
to the threshold voltage before the
input goes low again.  If the input stays high, then the capacitor is
allowed to charge fully, and the 555 timing interval ends, bringing
the output low.


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Previous: <a href="e-555schmitt.html">555 Schmitt Trigger (inverting)</a><br>
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<br>Generated Tue Dec  7 2010
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